1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming system and a method of detecting a color misregistration, and more particularly, to an image forming system that can detect color misregistrations of images formed on a transfer member, and a method of detecting the misregistrations performed in the above-described image forming system.
2. Discussion of the Related Art
A tandem-type configuration is widely used in related-art image forming apparatuses for producing color images. In such a related-art image forming apparatus employing a tandem-type configuration, single toner images of different colors formed on respective surfaces of multiple image carriers are sequentially overlaid to form a full color image. Such a related-art image forming apparatus includes an optical writing unit emitting a light beam according to image data and forming a latent image on each surface of the image carriers. The optical writing unit generally includes a polygon mirror for scanning and deflecting a light beam generated by a light source and multiple optical elements such as lenses and mirrors for detecting the light beam from the polygon mirror to form a latent image on a surface of an image carrier.
The above-described configuration of the optical writing unit is susceptible to deviation in positions and angles between the optical elements according to the following changes in the optical elements. That is, respective image forming surfaces of the optical elements may be curved, a housing of the optical writing unit may be distorted, various components forming the optical writing unit may be deformed by generation of heat by rotation of the polygon mirror, the image carriers may be deformed when mounted, and so forth.
When the above-described changes in positions and angles between the optical elements occur, a position of the scanning line of a light beam with respect to an image carrier may vary. In addition, curves and inclinations may be caused in the scanning line of the light beam on the surface of the image carrier. As a result, relative deviations in scanning position of the scanning line between the image carriers, the curves and inclinations of the scanning lines, and so forth may appear as color image misregistrations.
Therefore, related-art color image forming apparatuses employ optical sensors serving as image detection units to detect an amount of relative misregistration of the scanning line between adjacent image carriers, so that the color image misregistration can be properly corrected. After images are formed on a surface of a transfer member such as a transfer belt, the optical sensors detect the positions of the images. According to the detection results, the scanning position of the scanning line between the image carriers, the curves and inclinations of the scanning lines, and so forth are adjusted.
A speed of the transfer belt may vary due to eccentricity of a drive roller for driving the transfer belt, slight slippage of the drive roller and the transfer belt, shocks or impacts given to a recording medium when fed and/or discharged, a change of load by applying various biases such as a transfer bias, and so forth. Accordingly, the detection results of the optical sensors include elements of misregistration due to variations in speed of the transfer belt, which prevents the optical sensors from detecting accurately relative misregistrations of the scanning lines. Therefore, the scanning position of the scanning lines, the curves and inclinations of the scanning lines, and so forth cannot be accurately adjusted.
In a related-art color image forming apparatus, a distance from a transfer position of a photoconductor drum to an optical sensor serving as an image detection unit is set as an integer multiple of a travel distance of the transfer belt during one revolution of the drive roller. By so doing, a factor of the color image misregistration caused by variations in speed of the transfer member due to eccentricity of the drive roller at the transfer position can be removed when the optical sensors detect the images. As a result, even when the speed of the transfer belt varies due to eccentricity of the drive roller, the optical sensor can detect an amount of relative misregistration of the scanning line after the factor of the color image misregistration caused by the speed variation of the transfer member due to eccentricity of the drive roller is accounted for. Accordingly, the scanning position of the scanning line between the image carriers, the curves and inclinations of the scanning lines, and so forth can be adjusted accurately.
However, the above-described operation cannot remove other factors that are also causes of color image misregistration, such as a slight slippage of the drive roller and the transfer belt, shocks or impacts given to a recording medium when fed and/or discharged, a change of load by applying various biases such as a transfer bias, and so forth. These are the factors of the speed variations other than the speed variation of the transfer belt, one cycle of which is determined to be one revolution of the drive roller. Accordingly, even with the detection results obtained by the optical sensors, the scanning position of the scanning line between the image carriers and the curves and inclinations of the scanning lines are not effectively adjusted accurately.
A different related-art color image forming apparatus uses an encoder serving as a rotation detection unit. The encoder is mounted on a driven roller supportably extending a transfer belt to detect a rotation condition of the driven roller and control the speed of the transfer belt based on the detection results. This controls the speed variation of the transfer belt, such as the eccentricity of the drive roller, the slight slippage of the drive roller and the transfer belt, the shocks or impacts given to a recording medium when fed and/or discharged, the change of load by applying various biases such as a transfer bias, and so forth. As a result, from the detection results the amount of the relative misregistration of the scanning line of images formed on the transfer belt can be detected.
However, when the driven roller equipped with the encoder (hereinafter also referred to as an “encoder roller”) has eccentricity, feedback control is performed to the factor of speed variation caused by the eccentricity of the encoder roller, and therefore, the transfer belt may be affected by a speed variation due to eccentricity of the encoder roller. As a result, the image detection unit detects factors including the factor of misregistration of color images caused by the speed variation of the transfer belt due to eccentricity of the encoder roller. Accordingly, it is difficult to use the detection results of the images to detect the amount of relative misregistration of the scanning line accurately.